This invention relates to a panel of anisotropic material comprising acicular crystals enclosed in a vitreous or ceramic matrix, the crystals being orientated perpendicular to the panel faces and traversing the panel from one side to the other.
According to one method for manufacturing this panel, a homogeneous mineral composition capable of forming a ceramic glass is prepared from a mixture of mineral oxides and/or mineral compounds capable of generating such oxides, and a homogeneous layer of this composition in the vitreous state and of the shape and dimensions of the panel to be obtained is subjected to heat treatment consisting of bringing said layer to a temperature at least equal to its working temperature and greater than the crystallisation temperature of the crystalline phase, and establishing therein a thermal gradient perpendicular to its faces, and then progressively and gradually lowering the temperature of the layer starting from one of its faces, in such a manner as to successively bring each transverse plane of the layer to a temperature lower than the crystallisation range of the crystalline phase, so as to induce nucleation of the crystalline phase in that face plane which is at the lowest temperature followed by the growth of acicular crystals in said phase in the direction of the opposite face, while maintaining the thermal gradient such as to orientate the direction of maximum growth of the crystals perpendicular to the faces of the layer.
Depending on the nature of the crystals, the values of certain physical properties of the panel obtained by this method, such as its electrical or thermal conductivity, its magnetic susceptibility, its dielectric constant, its electro-optical and piezoelectric properties, etc. may be much higher in the direction perpendicular to its faces than in the direction parallel thereto.
This anisotropy may be utilised in various fields of applications of such a panel, notably in the manufacture of devices and apparatus which allow the visualisation and/or recording, in the form of an image, of information supplied in the form of electrical or magnetic signals, and in the manufacture of "memory" devices, designed for example for incorporation in electronic calculators, or in the manufacture of optical devices such as polarising filters or screens.
Anisotropic glass panels of this type are already known, as is their use as screens for cathode ray tubes, with the property of allowing the image formed by such a tube to be visualised and registered electrostatically on a suitable support.
One method for manufacturing these panels consists of mechanically assembling elements of different materials, such as metal needles or fibres and insulating glass. However, given that one of the main qualities which such panels must possess is a structural fineness sufficient to enable images formed by cathode ray tubes to be reproduced without loss of image definition, the conducting elements of the panels must be of very small diameter. It can therefore easily be conceived that their manufacture by this method is very difficult if not impossible to operate industrially.
U.S. Pat. No. 3,758,705 (Anthony P. Schmid) describes a process for manufacturing a glass panel comprising a large number of electrically conducting filiform crystals orientated perpendicularly to the faces of the panel and traversing this latter from one side to the other, the crystals being insulated one from the other by a non-conducting vitreous matrix.
This method consists of inducing nucleation of filiform crystals of reduced rutile, Ti.sub.x O.sub.2x-1, in a transverse plane of a mass of fused glass with the property of being able to be converted into a ceramic glass by the effect of appropriate heat treatment, the nucleation being triggered off by cooling the glass mass in the said plane to a convenient temperature, then making the crystals grow in this mass by gradually cooling adjacent transverse planes while maintaining a unidirectional thermal gradient parallel to the desired direction of growth of the crystals.
According to the U.S. Pat. No. 3,758,705, the mass of fused glass is placed in a refractory crucible, and nucleation of the rutile crystals is triggered off at the bottom of the mass by cooling the base of the crucible by a gas stream at ambient temperature. A vertical thermal gradient is thus created between the bottom of the glass mass and its upper free surface. Crystallisation of the rutile crystals is obtained while continuing to direct the gas stream onto the bottom of the crucible, so as to cause progressive cooling of the glass mass starting from the bottom.
To obtain elongated crystals constantly orientated perpendicularly to the faces of the glass panel and of regular structure from one to the other panel face, a constant cooling speed must be maintained appropriate to the speed of growth of the crystals, requiring the thermal gradient to be likewise maintained as constant as possible during the entire growth period.
The cooling method described in the U.S. Pat. No. 3,758,705 (gas stream directed onto the bottom of the crucible containing the fused homogeneous glass mass and, possibly, also on the upper free surface of the mass) is not very suitable for optimum adjustment of the cooling speed or for maintaining a constant thermal gradient during crystal growth.
In particular, according to the manner of operation of the method described in Example 2 of that Patent, the temperature of the bottom of the glass mass is firstly lowered below the lower limit of the crystallisation range of the crystalline phase starting from a temperature above this range, while maintaining the temperature of the top of the glass mass constant. The temperature of the top of the glass mass is then likewise lowered below the lower limit of the crystallisation range of the crystalline phase while maintaining the temperature of the bottom of the mass constant.
Thus the thermal gradient between the top and bottom of the glass mass does not remain constant during crystallisation of the crystalline phase, but increases during the first afore-mentioned period and then decreases, so that at the end of crystallisation it has almost returned to its original value.
Furthermore, in the above panel with rutile crystals, electrical conductivity is very unstable and varies widely, because it is dependent upon the non-stoichiometric state, i.e. Ti.sub.x O.sub.2x-1, which is produced from reducing TiO.sub.2. Therefore, in this case, the reducing atmosphere must be critically controlled to obtain an adequate electrical conductivity, and moreover nonstoichiometric solid of TiO.sub.2 such as Ti.sub.x O.sub.2x-1 is very unstable. Furthermore, the rutile crystals of such a panel are not ferroelectric as the products which will be proposed below, so that they cannot be applied for display devices utilizing polarisation phenomena.